1. Field of the Invention
The present invention relates to a water-glycol hydraulic fluid, or more particularly to a water-glycol hydraulic fluid having high wear resistance and excellent sludge solubility.
2. Description of the Prior Arts
In general, water-glycol hydraulic fluid is widely used as hydraulic fluid for various kinds of equipment such as hydraulic instruments since it is fire resistant or flame retardance. But it has a problem such that its wear resistance is unsatisfactory. It is known that an aliphatic acid can be mixed to the water-glycol hydraulic fluid in order to solve the problem. For example, Japanese Patent Publication No. 59159/1987 discloses that a higher unsaturated aliphatic acid and a higher saturated aliphatic acid are used in combination, and alkali hydroxide in an amount of more than the neutralization equivalent amount is mixed therein to prepare a water-glycol hydraulic fluid with improved wear resistance.
However, in the water-glycol hydraulic fluid with improved wear resistance by addition of a higher unsaturated aliphatic acid, the higher unsaturated aliphatic acid is liable to be deteriorated, and as a result, sludge solubility of the hydraulic fluid is lowered to precipitate a solid material dissolved therein in some cases.